Method of improving the resistance of alloy articles to corrosion



Patented Jan. 10,4933

UNITED STATES PATENT oFFzca i com a. rI x, or NEW YORK, N. x, ASSIGNORTO ALLEGHENY swam. comANY, or BRACKENBIDGE, rnNNsYLvANIA, A CORPORATIONor PENNSYLVANIA HEIHOD OF DJPROYING THE RESISTANCE OF ALLOY ARTICLES T0GORROSION 1T0 Drawing. Original application filed November 4, 1927,Serial No. 231,156. Divided and this application filed June 26, 1930.

This invention relates to a method of improving the resistance of alloyarticles to corrosion, and to the products of such method, and is adivision of my copending application, Serial No. 231,156 filed November4,1927.

By the present method, the resistance of articles made from variouscorrosion-resistant alloys is very largely improved. The inventionfurther provides a process which may be readily practiced, and by whichthe improved results are obtained in an economical manner. The inventionfurther provides a means of regenerating or recovering materials used inthe processes.

According to the present invention, an article made ofcorrosion-resistant alloy is treated in such a way as to dissolve fromthe surface the more soluble parts or elements of its surface, so as toleave a surface composed of the more resistant parts or elements of thealloy, and the pores or crystal interspaces are acked or filled with acompound of a metzfi capable of forming a corrosive-resistant oxid, insuch a manner as to stop or retard the progress of corrosion through thepores or crystal interspaces and thereby prevent the undermining of themore resistant parts or elements of the alloy left at the surface.

Theinvention is applicable generally to the treatment ofcorrosion-resistant alloys, and particularly to the alloys of iron withchromium and metals related to chormium, such as -molybdenum, vanadium,titanium, tungsten, manganese, cobalt and nickel.

The invention will be more particularly de-. scribed with reference toferro-chromium alloys, and particularly with reference to the so-calledlow chromium alloys containing, for example, 5 to 15 per cent chromium.

In carrying out the process; the more soluble parts of the alloy, theiron-rich constituents of ferro-chromium alloys for example, are removedby anacid which acts strongly upon such soluble parts and less stronglyor not at all upon the more resistant parts of the alloy. The commonmineral acids can be used, but sulphuric acid is preferred,particularlywith ferro-chromium alloys.

The packing of the pores or crystal inter- Serial No. 464,112.

spaces is effected by a solution containing a compound of a metal whichforms a corrosionresistant oxid. Suitable compounds are solublecompounds of chromium, vanadium,

tungsten, molybdenum, manganese, etc.

The articles are preferably subjected at one I immersion to the actionof the acid for'dissolving'the more soluble parts of the alloy and ofthe compound of the metal for packing the pores or crystal interspaces,the articles being immersed in a solution containing both pickle thearticles, so as to obtain a cleansurface, prior to treating the articlesto the action of the acid for selectively dissolving the surface, and tothe compound for packing the-crystal interspaces or pores.

A mode of procedure by way of example is as follows:

An article of a resistant alloy, as for example alow-chromium,ferro-chromium alloy tube, is first preferably pickled or cleaned. Asuitable pickling solution, for example, would be a solution of zincchloride and hydrochloric acid.

After the article has been cleaned,it is then placed in a bath ofsulphuric acid and chromic acid for selectively -dissolving theiron-rich parts of the surface metal and packing or filling the crystalinterspaces or pores.

Various concentrations of sulphuric acid and chromic acid may be used; Ihave found it preferable, however, to regulate the sulphuric acid toapproximately 7 per cent sulphuric acid ('6 cc. concentrated oleum in100 cc. water). It is preferable to have the chromic titanium, p

acid somewhat concentrated in the bath, as

pieces in such a solution from 40 .to 90'minfling immersing saidarticles in a 7% solution utes the loss in weight when subjected to a ofan acid, which is strong as regards the salt-spray test at 40 C. for90'hours is less iron-rich parts of the alloy, and of 60% than can beweighed, the test-pieces showlng chromium trioxid (chromic acid), andregenno signs of corrosion and retaining their erating said solutionwhen the iron content bright surface appearance. approximates 8O fgramsper liter.

The article after treatment according to 2, A th d 0 improving thresistance my process is one which-is very muc o of ferro-alloy articlesto corrosion, comprisresistant to corrosion than before treatment, ing ir i id ti l i a b th t i and in the example given, as stated above, ingabout 7% sulphuric-acid and about 60% samples do .not lose perceptiblyin weight chromic acid. a after being subjected to the salt-spray tes 3.A method of improving the resistance of at 40 C. for 90 hours. Bydissolving the ferro-alloy articles to corrosion, comprising ironrichconstituents out of the surface of forming a boiling bath containingabout 7% the alloy, the article is left with a Surface sulphuric acidand about 60% chromic acid, composed of chromium-ri h n tit n immersingsaid articles in said bath until the The undermining of thischromium-rich sursurfaces of said articles have had dissolved face isprevented by the chromic acid comout of them the iron-rich constituentswhich pounds which fill or pack'the pores and crysare replaced by acorrosion resistant chrotal interspaces. The chromic acid is the tr1-mium compound. oxide of chromium, and eventually converts .4. A methodof improving the resistance into the more stable sesqui-oxid Cr O or offerro-alloy articles to corrosion comprising corresponding compoundswith iron. The treating the surfaces of said articles with a corrosionresistant alloy articles form the sub bath of sulphuric acid and chromicacid to ject matter of my companion application filed dissolve thesoluble parts of the surfaces June 26, 1930, hearing serial number464,111. thereof and deposit chromium oxide in place In carrying out theprocess it has been of said dissolved parts, said bath containing foundthat the action of the chromic acid, 7% sulphuric acid and 60% chromicacid. sulphuric acid solution becomes sluggish 5. A method of improvingthe resistance when the concentration of the solution reaches offerro-alloy articles to corrosion comprising approximately 80 grams perliter of iron. treating the surfaces of said articles with a In order torender the solution usable and hot bath of sulphuric acid and chromicacid to recover the valuable constituents of the to dissolve the solubleparts of the surfaces bath, it is preferable to regenerate as folthereofand deposit chromium oxide in place lows: of said dissolved parts, saidbath containing The sluggish solution is led into a tank con 7%sulphuric acid and 60% chromic acid. taining a hot solution of halfsulphuric-acid Intestimony whereof, I have hereunto suband half water.The heat may be derived scribed my name this 21.day of June 1930. fromthe action of the sulphuric acid and COLIN G. FINK. water when addedtogether, or be otherwise supplied. In this solution the chromic acid isprecipitated as crystals of GrO the iron sulphate not being precipitatedimmediately from a hot solution. The chromic acid crystals areconveniently removed by filtering, or

by centrifuging. The commercial recovery exceeds 90% of the. chromicacid present. The recovered chromic acid may be redissolved in making upnew solutions or in strengthening solutions.

The sulphuric acid and iron sulphate dis- I solved or suspended thereinmay be separated by concentrating the sulphuric acid in an eva orator,for example, the iron sulphate precipltating in the concentratedsulphuric acid solution and settling as a sludge. The su rnatantsulphuric acid may be siphoned 0 or decanted,and used again in theprocess.

The invention may be carried out in various other modes of procedurethan that herein specifically described.

What I claim as new and desire to secure by Letters Patent is: v

1. A method of improving the resistance of ferro-alloy articles tocorrosion, compris-

